Hugo, it's not hard writing the callback, but why should I have to write one at all? WorldPay do not offer a solution where you can take all of a customer's details (including Credit Cards), submit it to them and have the transaction validated behind the scenes - other than using Microsoft COM or Java.

Instead, you have to take customer details, redirect them to the WP site, and then wait for a callback to find out whether the payment was successful. Callbacks aren't very robust. What if your web server goes down between taking the order but before the callback approval is received? WP callbacks don't keep "retrying." They get sent once (unless they've changed the system since we last used it). That means the customer has paid, but we have no acknowledgement of it

If you take an all or nothing system - we do everything at our end, including verifying the payment via a web service (SOAP, REST or whatever), either we've taken the order and validated the payment, or we've gone down (not nice, I know, but at least the order isn't in an unknown state)